Electronic devices, such as switch-mode power supplies (“SMPS”), when they operate, emit electromagnetic energy. This electromagnetic energy is often referred to as conducted emissions or electromagnetic interference (“EMI”), as it can interfere with the operation of other electronic devices. The emission of EMI by electronic devices is regulated. For example, the United States Federal Communications Commission (“FCC”) promulgates its Part 15 (47 C.F.R. §15) Class A requirements on conducted emissions, which specify EMI emissions requirements applicable to electronic devices to be used in commercial and industrial applications. The FCC also promulgates its Title 15 Class B requirements, which sets EMI emissions requirements for electronic devices to be used in residential applications. Standards organizations also publish recommendations for limits on conducted emissions. An example is standard numbers 16 and 22 published by the International Special Committee on Radio Interference of the International Electrotechnical Commission (“IEC”) (“CISPR 16” and “CISPR 22”), which specify conducted emissions limits and methods of measuring conducted emissions.
A SMPS includes one or more switches, such as transistors, that switch on and off at a controlled frequency. The switching on and off of the transistor controls the amount of energy that flows through the transistor in order to maintain a controlled voltage or current. One technique of controlling the switching of the transistor is to use pulse width modulation (“PWM”), where the period of the switching cycles is maintained constant and the width of the pulses is modulated, or varied, to adjust the amount of energy flowing through the transistor. The constant switching period of a PWM SMPS device, or the switching frequency of a variable-frequency switching device at steady state, can result in the emission of EMI that peaks at certain frequencies, such as the fundamental frequency of the SMPS device, and harmonics of those frequencies. Such peaks can exceed requirements and standards such as the FCC Class A or Class B, or CISPR 16 or 22. Component layout in an electronic device can also impact an electronic device's compliance with regulations. For example, components placed more closely to one another are more likely to interfere with one another, as the strength of the EMI decreases with distance.
Shielding the SMPS device, for example with a copper shield in order to prevent EMI from interfering with other devices, is costly. Moreover, the copper shield can be large relative to the circuit components, taking up significant space. What is needed is a way to reduce the emission of EMI by electronic devices, such as SMPS devices, and in particular PWM SMPS devices, in order to reduce their interference with other electronic devices and in some cases bring them into compliance with relevant requirements and standards.